A gamma camera (or scintillation camera) and a positron emission computed tomography position CT detector are known as examples of the systems which utilize position-sensitive radiation detectors. The gamma camera, as shown in FIG. 1, comprises a thin scintillator 70 of sodium iodide (NaI (TL) which is 5 to 15 mm in thickness and a light guide 71 disposed on the light output surface (lower surface) of the scintillator 70. Light emitted by the scintillator 70 is applied through the light guide 71 to a number of photomultipliers 72.
More specifically, the scintillator 70 emits light when excited by radiation, and the emitted light is applied through the light guide 71 to the photomultipliers 72 while spreading. The gamma camera further comprises a position calculating circuit 73 for determining an incident radiation position according to the ratio of light quantities distributed to the photomultipliers 72.
FIG. 2 shows a detector arrangement for use in the positron CT which includes a plurality of BGO scintillators 80 that are coupled through a light guide 81 to a plurality of photomultipliers 82. The detector further comprises a position calculating circuit 83 for detecting which scintillator has emitted light in response to incident radiation by comparing the outputs of the photomultipliers 82.
In the above-described gamma camera, the scintillator is relatively small in thickness. Therefore, in order to absorb high energy radiations effectively, it is necessary to increase the thickness of the scintillator. If the thickness of the scintillator is increased, however, the spatial spread of the light emitted by the scintillator upon application of the radiation is increased in the scintillator. In addition, it is necessary to increase further the spatial spread of the light in order to distribute the light to a suitable number of photomultipliers. Therefore, it is difficult for the gamma camera to have a high spatial resolution.
On the other hand, the detectors used in the positron CT detector uses a number of scintillators, as was described above. Since each scintillator is smaller in size than a photomultiplier, it is also necessary to increase the spatial spread of light to some extent. Therefore, this type detector also suffers from the difficulty of low spatial resolution. Furthermore, the detector in the positron CT detector is intricate in mechanical construction because it uses a number of photomultipliers.
In general, the spatial resolution (R) of a detecting system which adopts centroid calculation algorism in position detector, can be defined by the following expression: EQU R.varies.S/(N)1/2
where N is the quantity of light produced per radiation, and S is the spatial spread of the light produced. As is apparent from the expression, in order to decrease the positional resolution, it is necessary to decrease the spatial spread S, even if a centroid calculator is provided in a position detector.